1. Field of the Invention
This invention relates generally to circuit testing systems and more particularly to Kelvin-sensing systems.
2. Related Art
Kelvin connections (also known as “Kelvin sensing,” “four-terminal sensing,” “four-wire sensing,” or “four-point probes”) are often used in precision instrumentation applications to eliminate the effects of electrode impedance on measurement fidelity. In general, a Kelvin connection is an electrical impedance measuring technique that utilizes separate pairs of current-carrying and voltage-sensing electrodes to make more accurate measurements than traditional two-terminal sensing techniques.
As an example, if a user wishes to measure the resistance of some component located a significant distance away from an ohmmeter 100 as shown in FIG. 1 this scenario would be problematic because the ohmmeter 100 measures all resistance in the circuit loop 102 which includes the resistance of the wires (i.e., the electrodes from the ohmmeter 100 to the component), RWire 104 and 106, (also known as the force lead resistance, force lead impedance, electrode impedance, or impedance RForce) connecting the ohmmeter 100 to the component being measured, RSubject, 108 (also known as the “device under test” or “DUT”). In other words, the resistance measured by the ohmmeter 100 is equal to the combination of the resistances RWire 104, RSubject 108, and RWire 106.
In this example, the wire resistance may be very small (only a few ohms per hundreds of feet, depending primarily on the gauge (size) of the wire), but if the connecting wires (also known as the electrodes or force leads) 110 and 112 are very long and/or the component RSubject 108 to be measured has a very low resistance, the measurement error introduced by wire resistance RWire 104 and RWire 106 may be substantial.
Similarly, if instead of an ohmmeter 106, a user utilizes a power source (not shown) and voltmeter (not shown) to measure the voltage across (VMeasured 114) the combination of the resistances RWire 104, RSubject 108, and RWire 106, the measured voltage, VMeasured 114, includes the voltage drop across the force lead impedances RWire 104 and RWire 106 that result in voltage errors in the measured voltage, VMeasured 114.
The Kelvin sensing technique eliminates these problems by utilizing an ammeter and voltmeter with four terminals connected to the DUT. Since resistance is equal to the voltage divided by current, the resistance of the DUT may be determined by measuring the current going through it and the voltage dropped across it. Turning to FIG. 2, a typical known Kelvin connection is shown. In this example, an instrument 200 is shown that includes a power source 202, ammeter 204, and voltmeter (not shown) that measures a measured voltage, VMeasured 206, across a DUT 208. The instrument 200 is connected to the DUT 208 via a pair of force leads 210 and 212 (i.e., wires or electrodes) and sense leads (i.e., also wires or electrodes) 214 and 216. The force leads 210 and 212 include force impedance (i.e., wire resistance) values that are shown as RForce 218 and 220. In this example, the signal paths 210 and 212 are referred to as force leads because they are the signal paths along which a forced current 222 will flow from the power source 202 to the DUT 208 and back to the power source 202, where the forced current 222 is produced by the power source 202. The force leads 210 and 212 may also be interchangeably referred to as wires or electrodes. The signal paths 214 and 216 are referred to as sense leads because they are the signal paths that are utilized by voltmeter (not shown) to sense (i.e., measure) the measured voltage across the DUT 208. Similar to the force leads, the sense leads 214 and 216 may also be interchangeably referred to as wires or electrodes.
In an example of operation, the instrument 200 produces the forced current 222 with the power source 202 which is directed to DUT 208. The forced current 222 is the same at all points in the circuit because it is a series loop. Since the measured voltage, VMeasured 206, is across the DUT 208, this approach eliminates measuring any voltage drops across the force impedances, RForce 218 and 220, to produce a measured voltage, VMeasured 206, without any errors introduced by the force lead impedances, RForce 218 and 220.
Unfortunately, a problem with making measurements with known Kelvin-connected configurations is ensuring that the sense leads are properly connected to the DUT. This is of particular concern in complex automated test systems or systems utilizing fixtures with connectors or pogo pins, which are prone to occasional poor contact. Since the DUT voltage is generally not known, sense lead continuity cannot be reliably inferred from sense lead voltage measurements.
There have been a few known attempts to solve this problem. One approach is to force a DC current into the summing junctions of a differential amplifier that measures the sense lead voltage in a test instrument. Based on a measurement of the resultant voltage developed at the output of the differential amplifier, open sense leads (either or both) may be inferred, where the term “open sense lead” or “open sense lead condition” generally means a broken or discontinuous sense lead. However, this scheme only works when there is no voltage on the DUT, so the power source in the test instrument must be disabled during the detection process. This implies that continuity cannot be continuously verified throughout a test using this approach, but only before it begins or at specific times during the test when the power source is disabled. This approach is utilized by 66300-series of power supplies produced by Agilent Technologies, Inc., of Santa Clara, Calif.
In another approach, a system sends a transformer-coupled square current pulse (generated by a microprocessor upon user request) through each sense and force lead pair and compares the voltage response across each pair of wires to a threshold. However, this approach, similar to that described above, does not allow for continuous detection of an open sense lead condition. Moreover, this approach may be prone to creating observable pulses in a DUT voltage, measured sense voltage (especially when the force leads, sense leads, or both are long), or both. Additionally, this approach may be susceptible to false positive and false negative event detection in the event of coincidental large transient force lead voltages, which are generally caused by rapid changes in the force lead current. This approach is utilized by U.S. Pat. No. 5,886,530, titled “Test Contact Connection Checking Method and Circuit,” to Fasnacht et al.
An additional approach is described in U.S. patent application Ser. No. 2011/0309847, titled “High Current Kelvin Connection and Contact Resistance,” to Schwartz. In this approach, a system utilizes a center-tapped transformer stimulated by a square-wave voltage source (in a push-pull arrangement) to force currents through a pair of connections to verify continuity. The connections are coupled to the transformer using a switch that is closed when contact resistance measurements are being made. The primary of the transformer is center-tapped and the current through this center tap to common is measured, which allows inference of the contact resistance. As with the approaches described above, this scheme does not allow for detection of an open sense condition during testing, as the system would likely disrupt the test stimulus, measurement, or both.
As such, there is a need in the art for a system that allows for continuous broken sense lead detection in a Kelvin-connected instrument.